1. Field of the Invention
This invention relates, generally, to the field of detecting and/or monitoring chromium contamination and, more particularly, to a hexavalent chromium (“Cr(VI)”) monitor for detecting and/or continuously monitoring the concentration of Cr(VI) in a liquid such as water.
2. Background Art
Chromium and its compounds are primarily used in the manufacture of steel and other alloys, chrome plating, pigment production and leather tanning. In addition, chromate salts have been used for many years as excellent reagents in chemical laboratories. In the past, the hazardous characteristics of chromate compounds were not adequately recognized, so that chromium-containing waste was often inadequately disposed. At present, leaching of chromium compounds from waste sites to ground water has caused water contamination all around the world. Drinking water contamination has been reported in many places in the U.S.
Chromium can exist in nature as a compound in one of two stable valences. Chromium in trivalent chromium (“Cr(III)”) compounds is nontoxic and is actually an essential nutrient for the human body. Chromium in Cr(VI) compounds is known to be carcinogenic. Therefore, chromium contamination is actually a problem of Cr(VI) contamination. In water contamination investigations and contamination control, what is important is the concentration of Cr(VI) in the water.
Present laboratory analytical methods for chromium detection include, for example, inductively coupled plasma atomic emission spectrometry (“ICP-AES”), which can detect chromium in water to part-per-billion (ppb) levels. See, S. Tao and T. Kumamaru, Anal. Proc., 1995, 32, 371. Graphite furnace atomic absorption spectrometry (“GF-AAS”) also has the capability for detecting chromium in water to sub-ppb levels. See, R. E. Clement and P. W. Yang, Anal. Chem., 1999, 71, 257R. However, both the ICP-AES and GF-AAS methods can only give information about total chromium concentration in water. A separation procedure, such as solvent extraction, ion chromatography (“IC”) or high-pressure liquid chromatography (“HPLC”), must be employed in order to separate Cr(VI) from Cr(III) before detection by ICP-AES or GF-AAS. In addition, the instruments used for these analytical methods are expensive, large and susceptible to environmental noise. It is very difficult to deploy these instruments in field applications.
Ultraviolet visible light absorption spectrometry (“UV/VIS”) is a mature technique in analytical chemistry. In photometry and spectrometry, the composition and concentration of dissolved substances are determined by measuring the absorption of light in a liquid that includes such substances. These optical analysis techniques are based on the fact that different substances will absorb light at different wavelengths. See Z. Marczenko, ed., “SpectropHotometric Determination of Elements”, John Willey & Sons Inc., New York, 1976. There are two known UV/VIS methods for chromium detection. The first method is based on the reaction of chromate ions with diphenylcarbazide. The reaction produces a complex which absorbs light with peak absorption at 546 nm. The second method is based on the light absorbing property of the chromate ion itself. Chromate ions in aqueous solution can absorb light with a peak absorption at 373 nm.
Both of the known UV/VIS methods can selectively detect Cr(VI) compounds without interference from Cr(III) compounds. The problem with these methods for monitoring Cr(VI) water contamination is that their sensitivity is too low. The detection limit of the UV/VIS method with diphenylcarbazide as a reagent has been reported to be 20 ppb. The sensitivity of the UVVIS method based on light absorption by chromate ions is 40 times lower than that of diphenylcarbazide method. With present strict drinking water regulations requiring a maximum of 1 ppb Cr(VI) in drinking water, the application of these known methods is limited.
The sensitivity of an optical absorption spectrometric method depends on both the intrinsic property of the light absorber and the path length of the sample cell. According to the Beer-Lambert law, for an absorber in a specific solution, the sensitivity of the optical absorption method has a linear relationship with the path-length of the sample cell. See, D. A. Skoog, D. M. West and F. J. Holler, ed., “Fundamentals of Analytical Chemistry”, 7th edn., Saunders College Publishing, Fort Worth, 1996, pp. 506-519. The length of the sample cell in conventional UV/VIS absorption spectrometry is normally 1 cm. Sample cells of longer path length are possible, but a 10 cm path length is the limit in normal instruments. Several factors restrict the adoption of a longer path length sample cell in conventional UV/VIS spectrometry. These include the diffusion of the light beam in a long path length sample cell, the availability of intense light from a light source, the limit of the physical size of an instrument, the stray light noise, volume of sample required, etc.
Further, conventional UV/VIS instruments typically use a broadband light source with an optical dispersing element, such as a tungsten lamp coupled to a prism. While being suitable for an analytical laboratory, this type of instrument is unsuitable for field application as it is highly susceptible to damage in field operations.
While not limited thereto in its utility, the present invention has applicability to the field of fiber optics. Liquid core fiber-optic wave guides, i.e., light guide fibers in the form of a capillary filed with a liquid which functions as the light transmitting core, have previously been proposed. Consistent with the teachings of U.S. Pat. No. 3,894,788, the low index of refraction of water and other aqueous liquids rendered it impossible to employ such liquids as the light conducting core medium of a liquid-core, fiber-optic wave guide or the like. However, developments in material science has made long path length UV/VIS possible. Materials having a refractive index of less than water have been developed. For example, a special amorphous fluoropolymer, Teflon AF, developed and marketed by DuPont Fluoroproducts, has a refractive index from 1.29 (Teflon AF 2400) to 1.31 (Teflon AF 1600), which is smaller than that of water.
If a tube made from a material that has a refractive index of less than water is filled with water, the tube will behave as an optical fiber with water as the fiber core. See, R. Altkorn, I. Koev, R. P. Van Duyne and M. Litorja, Appl. Opt., 1997, 36, 8992. Light can be guided into such a water core fiber over a long distance with almost no diffusion loss and can be used as a sample cell in optical spectrometry. As will be appreciated, the interaction with light (absorption, fluorescence, scattering) of any species in the water that fills the tube can be detected.
For examples of such prior liquid core fiber-optic wave guides, reference may be made to U.S. Pat. No. 5,416,879 and U.S. Pat. No. 3,894,788. Further, U.S. Pat. No. 5,570,447 discloses the concept of a liquid core fiber optic waveguide cell for optical spectrometry. The liquid core optical fiber is an amorphous fluoropolymer coated tube filled with water. In U.S. Pat. No. 6,188,813B1, a long path length flow cell, which is an amorphous fluoropolymer tube filled with water, acts as an optical absorption detector for flow injection analysis. U.S. Pat. Nos. 6,011,882 and 6,016,372 disclose amorphous fluoropolymer tubes as sample cells for detecting an optical absorption, fluorescence, and chemiluminescence signal emitted from species diffused into the tube from its surrounding environment. Additionally, U.S. Pat. No. 6,332,049 B1 discloses an amorphous fluoropolymer tube as a sample cell for luminescence detection.